Question

You and your hypothetical lab partner each develop an experimental plan to test this hypothesis. The two experimental plans are presented below. Evaluate each plan for its pros and cons in the experimental design using the bullet points below. Both experimental plans have some good elements and areas that could be improved.

Experiment A) Mature 2 inch sections (sprigs) of Elodea will be used to measure oxygen production under different light conditions. 5 sprigs will be tested. One will be kept in the dark (no light). The others will be exposed to a light filtered with cellophane. One sprig’s light will be filtered through clear cellophane to receive all wavelengths of light, and the other sprigs’ light will be filtered through red, blue, and green cellophane, respectively. The no-light plant will be kept in a dark cabinet, while the other plants will be together on a counter. Each plant sprig will be placed in equal sized upside-down test tubes in 100ml of tap water so that oxygen bubbles produced by the plant will be trapped at the top of the test tube. A ruler will be used to measure the length of the oxygen pocket collected at the top of the test tube. The experiment will continue until one sprig produces 5 cm of oxygen in the tube.

Experiment B) 3 cm sections (sprigs) of Elodea of various ages will be used to measure oxygen production under different light conditions. 15 sprigs will be divided into 3 groups of 5 sprigs each. One group will receive natural sunlight, one group will be exposed to a blue light bulb under a box to block natural light, and the final group will be exposed to a red light bulb under a similar box to block natural light. Each group of plant sprigs will be trapped under a clear plastic funnel that is covered by an upside-down test tube to collect oxygen bubbles produced by all the sprigs in the group. All groups will be in 100ml of water of the same temperature and pH. The experiment will continue for 24 hours, and then the volume of oxygen gas produced will be measured by multiplying the length of the oxygen pocket by the circle described by the test tube, ?r². The average volume produced by all groups will be graphed and compared to determine which condition produced the most total volume of oxygen.

Consider the following questions to evaluate experimental plans A and B (DO NOT ANSWER THESE QESTIONS)

What is the independent variable suggested by the hypothesis? Do both plans test the independent variable equally well?

What is the dependent variable in each experiment? Which experiment’s dependent variable best matches the prediction in the hypothesis?

What are the experimental and control groups for each experiment? Which experiment gives the best opportunity to assess the effect of the independent variable?

How does each experiment account for standardized variables? Which experiment do you think is the better controlled experiment?

How is sample size addressed in each experiment? Which would you find to be more convincing?

Does either experiment describe any statistical analyses to be performed on the data?

ANSWER THIS QUESTION: BELOW

Considering your analysis of each of the questions above,write an improved experimental design to test the hypothesis “If blue light is more effective at promoting photosynthesis, then more oxygen gas (O₂) will be produced when plants are exposed to blue-filtered light compared to other wavelengths.”You may use elements of the above experiments

Answer 1

We tested the effects of blue and red light on the rate of plant photosynthesis. We hypothesized that light absorption by the plant and the energy level of different wavelengths of light are positively correlated to the rate of photosynthesis. Thus, because blue light has a higher absorbance by plant photosynthetic pigments and has a higher energy wavelength than red light, we predicted that juniper needles placed in blue light would photosynthesize faster than juniper needles placed in red light. We measured the rate of change in CO₂ concentration due to juniper needles. For each sample, we placed the needles into a chamber connected to the CO₂ monitor and measured the rate of change of CO₂ concentration for 10 minutes under red light and then 10 minutes under blue light. We ran three independent trials and alternated which color of light to which the leaves were first exposed. We weighed the juniper needles in each sample so that we could control for differences in mass; the rates of change of CO₂ concentration were calculated per gram of juniper needles. We did not test the rate of respiration of the juniper needles in the absence of light because we assumed that the rate of respiration was constant for each sample of juniper needles. We monitored the rate of change in CO₂ concentration of an empty chamber as a control to demonstrate that any change in CO₂ concentration was a result of the juniper leaves and not the chamber itself changing the concentration of CO₂. The rate of change of CO₂ concentration in the empty chamber was nearly 0, so we did not have to correct/adjust any values during the experiment due to this control. Plants in red light produced less CO2 over time (photosynthesized faster) than the plants in the blue light for each of our three trials. Two of the three trials in the red light were negative values, reflecting a decrease in the concentration of CO2. These values of the photosynthesis (plus respiration) rates in red light were 0.443, -0.141, and -1.1 ppm/g/min with a mean value of -0.27 ppm/g/min. The values of photosynthesis (plus respiration) rates in blue light were 2.449, 1.667, and 2.997 ppm/g/min with a mean value of 2.36 ppm/g/min. A t-test comparing the mean photosynthetic rates under red and blue light indicated no significant difference (p=0.068). However, this value is close to being significant, so with additional trials of our experiment it is possible that we would come up with a significantly faster rate of photosynthesis under red light compared to blue light. Based upon our results, we rejected our hypothesis. Blue light does not make plant needles photosynthesize faster than red light, and we see a trend towards faster rates of photosynthesis under the red light. Other student projects done in previous years produced similar results. One study found a decreasing rate of photosynthesis in blue light. Another study found that the rate of photosynthesis occurred fastest in red light and that the reason for this was because xanthophylls were dissipating the excess energy associated with blue light. One possible explanation for our results is that due to the high-energy nature of blue light, some of the blue light shining onto the juniper needles is absorbed by plant pigments other than the chlorophylls and is not transferred to the photosynthetic reactions. Xanthophylls and carotenes are possibly dissipating the high-energy blue light because xanthophylls and carotenes absorb only in the blue spectrum. These energy dissipation mechanisms operate in the blue spectrum because high energy blue light may be damaging to the plant. Further experimentation should be performed to verify our results and to test new hypotheses. In the future, more trials of our experiment should be run to test whether red light is photosynthesizing significantly faster than blue light. New experiments examining how and where blue light is absorbed by juniper needles are needed in order to better understand the effects of blue light on the plant.